On Jul 18, 7:51 am, Andrew Freeman <[EMAIL PROTECTED]> wrote: > Andrew Freeman wrote: > > oj wrote: > >> On Jul 18, 12:10 pm, John Machin <[EMAIL PROTECTED]> wrote: > > >>> On Jul 18, 9:05 pm, oj <[EMAIL PROTECTED]> wrote: > > >>>> On Jul 18, 11:33 am, [EMAIL PROTECTED] wrote: > > >>>>> Hi, > >>>>> Hi, > >>>>> I am taking a string as an input from the user and it > >>>>> should only > >>>>> contain the chars:L , M or R > >>>>> I tried the folllowing in kodos but they are still not > >>>>> perfect: > >>>>> [^A-K,^N-Q,^S-Z,^0-9] > >>>>> [L][M][R] > >>>>> [LRM]?L?[LRM]? etc but they do not exactly meet what I need. > >>>>> For eg: LRLRLRLRLM is ok but LRLRLRNL is not as it has 'N' > >>>>> .like that. > >>>>> regards, > >>>>> SZ > >>>>> The string may or may not have all the three chars. > > >>>> With regular expressions, [^LRM] matches a character that isn't L, R > >>>> or M. So: > >>>> import re > >>>> var = "LRLRLRLNR" > >>>> if re.search(r'[^LRM]', var): > >>>> print "Invalid" > > >>> Fails if var refers to the empty string. > > >> No it doesn't, it succeeds if var is an empty string. An empty string > >> doesn't contain characters that are not L, R or M. > > >> The OP doesn't specify whether an empty string is valid or not. My > >> interpretation was that an empty string would be valid. > > > Why not just use * instead of + like: > > > if re.search(r'^[^LRM]*$', var): # note: ^ outside [] is start of > > string; $ means end of string > > print "Invalid" > > > This will *only* print invalid when there is a character other than L, > > R, or M or a empty string. > > Sorry, forget the beginning and ending markers, I just tried it out, it > doesn't work. > use this instead: > > if re.search(r'[^LRM]*', var): > print "Invalid"
This won't work -- every string in the universe contains 0 or more characters which are not 'L', 'R', or 'M'. That is, the regular expression X* could match the empty string, which can be found in all strings. -- http://mail.python.org/mailman/listinfo/python-list
